Effect of inhaled carbon dioxide on laryngeal abduction

Influence of doxapram and intermittent 10% carbon dioxide inspiration on cardiovascular and laryngeal functions in anesthetized dogs

OBJECTIVE

To compare the effects of two doses of doxapram intravenous injection and carbon dioxide inhalation on the cardiovascular and laryngeal functions of anesthetized hounds.

STUDY DESIGN

Experimental study.

ANIMALS

Six healthy adult dogs.

METHODS

In a Latin-square design, the mean arterial blood pressure (MABP) and heart rate (HR) were recorded continuously. The inspiratory normalized glottic gap areas (iNGGA) were measured before and after each stimulation with 0.55 mg/kg of doxapram (L-DOX), 2.2 mg/kg of doxapram (H-DOX), or 90 s of inhalation of 10% carbon dioxide in oxygen (I-CO₂ ). The stimulations were tested in duplicate or triplicate. Video clips of the laryngeal movement were scored by board-certified surgeons masked to the treatment.

RESULTS

The MABP increased with L-DOX and H-DOX up to 81% (both p < .001 compared to I-CO₂ ), and persisted during the other stimulations (both p < .001). An intermittent tachycardic effect of up to 79% increase in HR was observed with doxapram. The HR following H-DOX was higher than L-DOX and I-CO₂ (both p < .016). Neither hypertension nor tachycardia was observed with I-CO₂ . The iNGGA increased with all treatments (p < .001). The iNGGA was greater with H-DOX than L-DOX and I-CO₂ (both p < .007). All treatments received higher scores (all p < .001) with acceptable inter- and intra-observers Krippendorff's alphas.

CONCLUSION

All treatments were effective respiratory stimulants in anesthetized dogs; however, doxapram caused hypertension and tachycardia.

CLINICAL SIGNIFICANCE

Carbon dioxide inhalation might improve arytenoid motion without cardiovascular effects in dogs during clinical airway examinations.

Laryngeal function in normal dogs administered isoflurane following partial clearance of alfaxalone or propofol

OBJECTIVE

To assess laryngeal function in normal dogs administered isoflurane following partial clearance of alfaxalone or propofol.

STUDY DESIGN

Randomized experimental crossover study.

ANIMALS

A group of 12 purpose-bred, male Beagle dogs.

METHODS

Dogs were randomly assigned to one of two treatments: alfaxalone-isoflurane (ALF-ISO) or propofol-isoflurane (PRO-ISO) and anesthetized for three video laryngoscopy examinations. The alternate treatment occurred after ≥ 14 days interval. Examinations were performed after induction of anesthesia (LS-A), after 20 minutes of breathing isoflurane via a facemask (LS-B) and after a further 20 minutes of isoflurane (LS-C). Parameters of objective laryngeal function included inspiratory rima glottidis surface area (RGSA-I), expiratory rima glottidis surface area (RGSA-E) and % RGSA increase, calculated from three consecutive respiratory cycles in the final 15 seconds of each video laryngoscopy examination. The % RGSA increase was calculated using [(RGSA-I - RGSA-E)/RGSA-E] × 100. Subjective laryngeal function was evaluated independently by two experienced surgeons blinded to treatment.

RESULTS

The % RGSA increase within each treatment was greater for LS-B and LS-C than for LS-A (ALF-ISO: p = 0.03, PRO-ISO: p = < 0.001). There was no difference within each treatment from LS-B compared with LS-C. RGSA-I increased within each treatment from LS-A to both LS-B and LS-C (ALF-ISO: p = 0.002) and to LS-C (PRO-ISO: p = 0.006). Subjective laryngeal function scores improved from LS-A to LS-C.

CONCLUSIONS AND CLINICAL RELEVANCE

Laryngeal function improved from postinduction examination following either 20 or 40 minutes of anesthesia with isoflurane via facemask. This study demonstrates that isoflurane may have a lesser effect on arytenoid abduction activity compared with more commonly used intravenous induction anesthetics (alfaxalone and propofol).

Muscles of Breathing: Development, Function, and Patterns of Activation

This review is a comprehensive description of all muscles that assist lung inflation or deflation in any way. The developmental origin, anatomical orientation, mechanical action, innervation, and pattern of activation are described for each respiratory muscle fulfilling this broad definition. In addition, the circumstances in which each muscle is called upon to assist ventilation are discussed. The number of "respiratory" muscles is large, and the coordination of respiratory muscles with "nonrespiratory" muscles and in nonrespiratory activities is complex-commensurate with the diversity of activities that humans pursue, including sleep (8.27). The capacity for speech and adoption of the bipedal posture in human evolution has resulted in patterns of respiratory muscle activation that differ significantly from most other animals. A disproportionate number of respiratory muscles affect the nose, mouth, pharynx, and larynx, reflecting the vital importance of coordinated muscle activity to control upper airway patency during both wakefulness and sleep. The upright posture has freed the hands from locomotor functions, but the evolutionary history and ontogeny of forelimb muscles pervades the patterns of activation and the forces generated by these muscles during breathing. The distinction between respiratory and nonrespiratory muscles is artificial, as many "nonrespiratory" muscles can augment breathing under conditions of high ventilator demand. Understanding the ontogeny, innervation, activation patterns, and functions of respiratory muscles is clinically useful, particularly in sleep medicine. Detailed explorations of how the nervous system controls the multiple muscles required for successful completion of respiratory behaviors will continue to be a fruitful area of investigation. © 2019 American Physiological Society. Compr Physiol 9:1025-1080, 2019.

Partial neuromuscular block impairs arytenoid abduction during hypercarbic challenge in anesthetized dogs

OBJECTIVE

To evaluate the effect of two levels of partial neuromuscular block (NMB) on arytenoid abduction, tidal volume (V_T) and peak inspiratory flow (PIF) in response to a hypercarbic challenge in anesthetized dogs.

STUDY DESIGN

Prospective laboratory study.

ANIMALS

Eleven healthy male Beagle dogs aged 3-5 years.

METHODS

Dogs were anesthetized with propofol and dexmedetomidine infusions. The rima glottidis was observed via an endoscope placed through a laryngeal mask airway. Atracurium infusion was titrated to obtain two levels of partial NMB. The normalized glottal gap area (NGGA; glottal gap area normalized to height squared of rima glottidis) at peak inspiration during a hypercarbic challenge (10% CO₂ inspired for 1 minute) was measured at baseline, during mild [train-of-four (TOF) ratio 0.4-0.6] and shallow (TOF ratio 0.7-0.9) NMB, and 30 minutes after spontaneous recovery from NMB. The V_T and PIF were measured at the same time points and compared using anova for repeated measures and Tukey's post hoc tests.

RESULTS

The NGGA and V_T were significantly lower than baseline during both levels of partial NMB with no difference between mild and shallow NMB (p < 0.05). They returned to baseline values after spontaneous recovery from NMB. PIF was not altered significantly during partial NMB.

CONCLUSIONS AND CLINICAL RELEVANCE

The NGGA and V_T at peak inspiration in response to a hypercarbic challenge were reduced during partial NMB block, with decreased abduction of the arytenoid cartilages. This dysfunction was present even at shallow levels of NMB.